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Daily Archives: 4 March, 2008

So far we’ve covered going faster and we’ve covered going around corners. The last essential ingredient is of course, stopping (or just slowing down). Motorcycles use two separate hydraulic braking systems which operate independently of each other and (for most cases) affect one wheel each. The front brake is controlled by a hand-lever on the right hand side whilst the rear wheel is controlled by a pedal activated by the rider’s right foot.

Ironically, my current motorcycle (a Honda VFR 800) uses a system that sends a differing proportion of the braking force to both wheels when using either the brake lever or pedal. It does this in an attempt to make braking a safer venture than it may normally be in the hands of an unskilled operator. The weight transference (towards the front of the motorcycle) that occurs when braking, adversely affects the amount of braking force that can carried out effectively by each wheel. Some figures suggest in dry conditions as much as 90% of the braking force can be delivered via the front wheel. The “linked brakes” of the VFR are Honda’s solution to removing this judgement from the rider.

When you look at the contact patch that the front wheel has on the ground, you begin to realise that there is a lot of momentum being shed through a very small area. Motorcycle training will teach riders that they need to “set up” their braking: transferring weight progressively to the front and thereby compressing the suspension and tyre gently. As this weight transfer occurs, the tyre is flattened out on the ground, increasing the size of the contact patch. This in turn allows more force to be applied in a controlled manner.

There is a theory in physics known as the “Conservation of Energy”. It states that “energy can neither be created nor destroyed. – It can only be converted from one form to another”. A motorcycle, or indeed any mass when moving is said to have “kinetic energy”. The faster it goes, the more kinetic energy it has. Therefore, stopping a motorcycle reduces the amount of kinetic energy the bike has. But, because of the “conservation of energy”, we know that this energy hasn’t been lost. What has happened to it? Chiefly, it has been converted into heat energy. – That’s what brakes do, they turn kinetic energy into heat energy. This heat energy is then dissipated through both the air and the braking components, thus doing its own little bit to help keep the planet warm…

Now the astute amongst you may be thinking along the lines of “it takes a lot of power to accelerate a motorcycle quickly, how can I generate the strength required to stop it as quickly, simply by squeezing the brake lever?” If this thought has crossed your mind: Well done! It shows you’ve been paying attention… The answer lies in the fact that you are utilising a hydraulic brake system. In cars and some top-end motorcycles featuring ABS systems, the brakes include a mechanical/electrical system to increase the force you can apply to the brakes yourself. I’m not going to go into how these systems work, rather, I’ll stick to a plain-vanilla style brake set up found on most “conventional” motorcycles.

Hydraulics work on the principal that you can’t compress liquid. In our case this liquid is brake fluid. At the lever (or pedal) end, moving the level pushes a piston, which in turn pushes the liquid through the brake line(s). At the other end of the brake line is the “brake caliper” which contains one or more pistons of its own. With nowhere else for the liquid to go, these pistons are now displaced too, which pushes the brake pad onto the brake disk. The disk is attached to the wheel, and so is rotating, whereas the pads and caliper are fixed. When the disk and pads come into contact, there is friction which converts the kinetic energy into heat energy and “voilà!” you are slowing down! (Hopefully slowing fast enough to avoid a sudden impact with the scenery…)

This still doesn’t explain how you manage to provide enough force for the brake pads to grip the disk with the necessary bite to stop. Well, the really cool thing about hydraulics is known as “Hydraulic Multiplication”. If you change the size of the piston at one end, you can increase the force this piston pushes with. If this sounds too good to be true, it isn’t… Although you are gaining more force, the distance you are moving the piston at the other end is reduced. Fortunately for us, we don’t have to move the brake pads very far to make them grip the disk. For a more in depth look at how hydraulics work, you may want to look at the brilliant “How stuff works” page.